Conventionally, there is a known network session system that enables musical sessions such as musical performance (ensemble) played with musical instruments and chorus such as duet via a wide-area communication network such as the Internet. This session system is designed such that each terminal transmits musical performance information generated in accordance with musical performance played on the each terminal to a partner terminal via the wide-area communication network so that the respective terminals can generate musical sounds played by both the terminals. However, the conventional session system is disadvantageous in that in a case where the terminals are located behind their NAT (network address translation) routers, respectively (on the local side), it is necessary for the terminals to traverse the NAT routers in order to directly exchange musical performance information between the terminals without a server.
The NAT technology is described in later-described Non Patent Literature 1, “RFC 4787”. The RFC 4787 is technical specifications disclosed by IETF (Internet Engineering Task Force) to describe NAT properties related to NAT router traversal on unicast UDP. As behaviors of NAT routers, three patterns of Endpoint-Independent Mapping, Address-Dependent Mapping, and Address and Port Number-Dependent Mapping are commonly well known. Hereafter, these three behavior patterns of NAT routers will be described. FIGS. 1A to 1C illustrate behavior patterns of NAT routers. In a case where a source terminal such as a PC (personal computer) located behind the NAT router transmit packets by use of the same combination “source address: 192.168.0.1”, and “source port number: 5000”, the source address and source port number are to be translated by the NAT routers of the three different patterns as indicated in FIGS. 1A to 1C.